Distortion compensator, optical receiver, distortion compensator and optical receiver controlling methods, and optical transmission system

ABSTRACT

A distortion compensator, an optical receiver and a transmission system including an operation selectively compensating for linear waveform distortion exerted on an optical signal via a plurality of distortion compensators and compensating for nonlinear waveform distortion exerted on the optical signal using nonlinear distortion compensators.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2008-211196, filed on Aug. 19,2008, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to a distortion compensator, an opticalreceiver, distortion compensator and optical receiver controllingmethods and an optical transmission system. More specifically, thepresent invention includes a technique of compensating for waveformdistortion generated in an optical transmission line and a deviceimplementing same.

2. Description of the Related Art

In communication networks, optical communication using an optical fiberas an optical transmission line is being frequently used. In order tocope with a recently promoted increase in volume of information flowingthrough a network, increases in distance and capacity of an opticaltransmission system used are now being requested. However, waveformdistortion exerted on light in an optical fiber is one of factors thatrestricts the increases in transmission distance and capacity of anoptical transmission system. The waveform distortion exerted on light isgenerally classified into linear distortion and nonlinear distortion.Primary chromatic dispersion and primary polarization mode dispersionare included in the linear distortion. Both the linear distortion andthe nonlinear distortion are compensated for in an optical receiver thatreceives an optical signal from an optical transmission line. The lineardistortion may be compensated for by using a digital coherent techniquein an optical receiver that receives an optical signal which has beentransmitted through an optical fiber. Self phase modulation is includedin the nonlinear distortion. Compensation for nonlinear distortion whichis realized in an optical receiver is described, for example, in“Optical Express 16, pp 889-896 (2008), by Kazuo Kikuchi”.

According to the technique described in the above mentioned literature,nonlinear distortion generated in an optical fiber is compensated for inan optical receiver. However, as will be described in detail below, theaccuracy in compensation is limited.

SUMMARY

According to embodiment(s) of the present invention, there are provideda distortion compensator and an optical receiver. Each of the distortioncompensator and the optical receiver is of a type inputting thereinto anelectric signal obtained by photoelectric-converting an optical signalreceived from an optical transmission line and includes a configurationin which a plurality of distortion compensating sections (distortioncompensators). Each has linear distortion compensating sections (lineardistortion compensators) compensating for linear waveform distortionexerted on the optical signal and nonlinear distortion compensatingsections (nonlinear distortion compensators) compensating for nonlinearwaveform distortion exerted on the optical signal, are cascade-connectedwith one another.

According to embodiment(s) of the present invention, there are alsoprovided a distortion compensator controlling method and an opticalreceiver controlling method, each including a distortion compensatingoperation having a linear distortion compensating operation compensatingfor linear waveform distortion exerted on an optical signal and anonlinear distortion compensating operation compensating for nonlinearwaveform distortion exerted on the optical signal respectively based onan electric signal obtained by photoelectric-converting the opticalsignal received from an optical transmission line, and an executingoperation executing the distortion compensating operation a plurality oftimes.

According to embodiment(s) of the present invention, there are furtherprovided a distortion compensator controlling method and an opticalreceiver controlling method, each including a distortion compensatingoperation having an operation compensating for linear waveformdistortion exerted on an optical signal and an operation compensatingfor nonlinear waveform distortion exerted on the optical signalrespectively based on an electric signal obtained byphotoelectric-converting the optical signal received from an opticaltransmission line and information obtained from an optical transmissionline network control unit, and an executing operation executing thedistortion compensating operation a plurality of times.

According to an embodiment of the present invention, there is furtherprovided an optical transmission system controlling the operation of thedistortion compensator by the above mentioned distortion compensatorcontrolling method and controlling the operation of the optical receiverby the above mentioned optical receiver controlling method.

Additional aspects and/or advantages will be set forth in part in thedescription which follows and, in part, will be apparent from thedescription, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages will become apparent and morereadily appreciated from the following description of the embodiments,taken in conjunction with the accompanying drawings of which:

FIG. 1 is a block diagram illustrating an optical transmission systemaccording to an embodiment of the present invention;

FIG. 2 is a block diagram illustrating a transmission span;

FIG. 3 is a schematic diagram illustrating chromatic dispersion relativeto a transmission distance of an optical transmission system;

FIG. 4 is a schematic diagram illustrating optical signal power relativeto a transmission distance of an optical transmission system;

FIG. 5 is a block diagram illustrating an optical reception device;

FIG. 6 is a block diagram illustrating a nonlinear distortioncompensating section;

FIG. 7 is a diagram illustrating optical signal power in a transmissionspan;

FIG. 8 is a block diagram illustrating a multistage distortioncompensating section according to an embodiment of the presentinvention;

FIG. 9 is a block diagram illustrating one example of a lineardistortion compensating section according to an embodiment of thepresent invention;

FIG. 10 is a block diagram illustrating another example of a lineardistortion compensating section;

FIG. 11 is a block diagram illustrating an example of a nonlineardistortion compensating section according to an embodiment of thepresent invention;

FIG. 12 is a block diagram illustrating a digital processing section;

FIG. 13 is a constellation diagram illustrating nonlinear distortion;

FIG. 14 is a block diagram illustrating an optical reception deviceaccording to an embodiment of the present invention;

FIG. 15A is a block diagram illustrating an optical reception deviceaccording to an embodiment of the present invention;

FIG. 15B is a block diagram illustrating an optical reception deviceaccording to an embodiment of the present invention;

FIG. 16A is a block diagram illustrating an optical reception deviceaccording to an embodiment of the present invention;

FIG. 16B is a block diagram illustrating the optical reception deviceaccording to an embodiment of the present invention;

FIG. 17A is a block diagram illustrating an optical reception deviceaccording to an embodiment of the present invention;

FIG. 17B is a block diagram illustrating the optical reception deviceaccording to an embodiment of the present invention

FIG. 18 is a block diagram illustrating a distortion compensatingsection according to an embodiment of the present invention;

FIG. 19 is a block diagram illustrating a nonlinear distortioncompensating section according to an embodiment of the presentinvention;

FIG. 20 is a block diagram illustrating a transmission system accordingto an embodiment of the present invention;

FIG. 21 is a block diagram illustrating flows of respective pieces ofinformation; and

FIG. 22 is a diagram illustrating a processing method performed using aline card control unit.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to the embodiments, examples ofwhich are illustrated in the accompanying drawings, wherein likereference numerals refer to the like elements throughout. Theembodiments are described below to explain the present invention byreferring to the figures.

Next, preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

FIG. 1 is a block diagram illustrating an optical transmission system.In the optical transmission system illustrated in FIG. 1, an opticaltransmission device 112 converts an electric signal to an optical signaland outputs the converted optical signal to an optical transmissionline. The optical transmission line has a plurality of cascade-connectedtransmission spans 100. The transmission span 100 at the first stagereceives the optical signal sent from the optical transmission device112. The optical signal is propagated through a plurality of stages oftransmission spans 100 which are installed in the midst of thetransmission line. Then, the transmission span 100 at the final stagepropagates the optical signal to an optical reception device (opticalreceiver) 110. The optical receiving device 110 converts the opticalsignal to an electric signal and outputs the converted electric signal.

FIG. 2 is a block diagram illustrating an example of the transmissionspan 100. One transmission span 100 includes a transmission line 90,optical amplifiers 92 and 96, and a chromatic dispersion compensatingmodule 94. The transmission line 90 is, for example, an optical fiber.The optical amplifiers 92 and 96 are, for example, rare-earth addedfiber optic amplifiers or Raman amplifiers and amplify an optical signalwhich has been attenuated in the transmission line 90. The chromaticdispersion compensating module 94 compensates for chromatic dispersiongenerated in the transmission line 90.

FIG. 3 is a schematic diagram illustrating chromatic dispersion relativeto a transmission distance of an optical transmission system. The morethe transmission distance over which the optical signal is transmittedthrough the transmission line 90 is increased, the more the chromaticdispersion is increased. The chromatic dispersion compensating module 94compensates for the chromatic dispersion so increased. FIG. 4 is aschematic diagram illustrating optical signal power relative to atransmission distance. In FIG. 4, the vertical axis indicates astandardized logarithm of the optical signal power. The more thetransmission distance over which the optical signal is transmittedthrough the transmission line 90 is increased, the more the opticalsignal power is attenuated. The optical amplifiers 92 and 96 amplify anoptical signal of one wavelength or a wavelength-multiplexed opticalsignal to compensate for the attenuated optical signal power. For theconvenience of explanation, FIGS. 3 and 4 illustrate examples in whichit is assumed that all the transmission spans 100 exhibit the samechromatic dispersion value and the optical signal power value. However,in an actually installed optical transmission system, the characteristicand a length of an optical fiber of the transmission line 90, an amountby which a signal is chromatic-dispersed (hereinafter, referred to as achromatic dispersion amount) and an amount by which a signal isattenuated (hereinafter, referred to as an attenuation amount) in thetransmission line 90, an amount by which chromatic dispersion iscompensated for (hereinafter, referred to as a chromatic dispersioncompensation amount) of the chromatic dispersion compensating module 94and amounts by which the optical amplifiers 92 and 96 amplify signals(hereinafter, referred to as amplification amounts) of the opticalamplifiers 92 and 96 may be different for different transmission spans100.

Next, an optical reception device (optical receiver) performingnonlinear distortion compensation explained in the above mentionedliterature, that is, “Optical Express 16, pp 889 to 896 (2008)” will bedescribed as a comparative example. FIG. 5 is a block diagram of anoptical reception device (optical receiver) as the comparative example.In FIG. 5, the optical reception device 110 inputs thereinto an opticalsignal. A photoelectric converter (O/E) 12 converts the optical signalto an electric signal. An ADC (Analog Digital Converter) 14 converts theanalog electric signal to a digital electric signal. The digitalelectric signal is processed using a digital processing section 50. Thedigital processing section 50 has a chromatic dispersion compensatingsection (a chromatic dispersion compensator) 80, a nonlinear distortioncompensating section (a nonlinear distortion compensator) 82 and asignal processing section 10. The chromatic dispersion compensatingsection 80 compensates for linear waveform distortion which has beengenerated in the transmission span 100. The nonlinear distortioncompensating section 82 compensates for nonlinear waveform distortionwhich has been generated in the transmission span 100. The signalprocessing section 10 performs signal processing such as frequencyoffset compensation and phase synchronization on an electric signalwhich has been subjected to distortion compensation and outputs theelectric signal so subjected to signal processing.

FIG. 6 is a functional block diagram illustrating an example of thenonlinear distortion compensating section 82. The nonlinear distortioncompensating section 82 has a function of compensating for nonlineardistortion such as self phase modulation. The self phase modulation isnonlinear distortion of the type that when the power of an opticalsignal is increased in an optical fiber, phase modulation is induced inaccordance with the optical signal power so increased. Thus, anintensity monitor 70 monitors the intensity of an electric signalcorresponding to the optical signal so increased in power. An amplifier74 amplifies an intensity signal sent from the intensity monitor 70 andoutputs the amplified intensity signal to a phase modulating section 72.The phase modulating section 72 performs phase modulation in accordancewith the intensity signal so as to compensate for the nonlinear waveformdistortion generated in the transmission span 100. The phase modulatingsection 72 may perform phase modulation including according to anequation (5) described in the above mentioned literature. In the abovementioned manner, the self phase modulation generated in thetransmission span 100 is compensated for.

However, distortion compensation realized using the optical receptiondevice described in the comparative example is insufficient. FIG. 7illustrates a relation among optical signal power, distortion effect anddistortion compensation in a transmission span 100. In the drawing, thehorizontal axis indicates a direction in which an optical signal istransmitted and the vertical axis indicates the optical signal power. Inthe transmission line 90 illustrated in FIG. 7, as the optical signalpower is uniformly attenuated, linear distortion effect (for example,chromatic dispersion) is uniformly generated. Nonlinear distortioneffect such as the self phase modulation is generated when the opticalsignal power is high. In a region 91 on the input side of thetransmission line 90, the optical signal power is high, so that the rateat which both the linear distortion effect and the nonlinear effect aregenerated is high. The optical amplifier 92 amplifies the optical signalpower. The chromatic dispersion compensating module 94 attenuates theoptical signal power and compensates for the linear distortion effect.The optical signal power is high in a region 95 on the input side of thechromatic dispersion compensating module 94 and hence nonlineardistortion effect is generated. The optical amplifier 96 amplifies theoptical signal power.

As described above, in the actually installed optical transmissionsystem, combined effect of nonlinear distortion effect with lineardistortion effect and single effect of the linear distortion effect arealternately generated. Therefore, compensation for distortion, inparticular, compensation for nonlinear distortion may not be performedwith accuracy by a method in which linear distortion generated in aplurality of spans 100 is compensated for at one time and then nonlineardistortion is compensated for as in the comparative example illustratedin FIG. 5. An optical reception device (optical receiver) and an opticalreceiving method according to embodiment(s) of the present inventionhave been made in view of the above mentioned circumstances.

FIG. 8 is a block diagram illustrating an example of a distortioncompensator 20 installed in the optical reception device according to anembodiment of the present invention. The distortion compensator 20 isinstalled in place of the chromatic dispersion compensating section 80and nonlinear distortion compensating section 82 illustrated in FIG. 5.The distortion compensator 20 has a plurality of linear distortioncompensating sections (linear distortion compensators) 22 and aplurality of nonlinear distortion compensating sections 24. One lineardistortion compensating section 22 is paired with one nonlineardistortion compensating section 24 to form a one-stage distortioncompensating section (distortion compensator) 25. The distortioncompensator 20 has a configuration in which a plurality of (multistage)distortion compensating sections 25 are cascade-connected with oneanother. Thus, in the following description, the distortion compensator20 will be referred to as a multistage distortion compensating section(distortion compensator) as the case may be. The plurality of distortioncompensating sections 25 are cascade-connected with one another in amultistage form, so that the linear distortion compensating sections 22and the nonlinear compensating sections 24 are alternately arranged.That is, the multistage compensating section 20 alternately performslinear distortion compensation and nonlinear distortion compensation ona signal which has been input thereinto and outputs a signal sosubjected to distortion compensation. A control section 30 controls theoperations of the linear distortion compensating sections 22 and thenonlinear distortion compensating sections 24 to optimize laterdescribed coefficients. As an alternative, a configuration including thedistortion compensator 20 and the control section 30 may be defined as adistortion compensator. In other words, the distortion compensator has aconfiguration having the multistage distortion compensating section 20and the control section 30.

As described in relation to the comparative example illustrated in FIG.7, the combined effect of nonlinear distortion effect with lineardistortion effect and single effect of linear distortion effect arealternately generated in the transmission span 100. Therefore, highlyaccurate distortion compensation is realized by alternately performingcompensation for the combined effect of nonlinear distortion effect withlinear distortion effect and compensation for the single effect oflinear distortion effect as illustrated in FIG. 8.

For example, in FIG. 7, in the case that the intensity of the opticalsignal to be input into the chromatic dispersion compensating module 94is not high, nonlinear distortion effect is generated on the input side91 of the transmission line 90 in one transmission span 100. In thetransmission system in which a plurality of the transmission spans 100of the above mentioned type are connected with one another, the lineardistortion compensating sections 22 and the nonlinear distortioncompensating sections 24 are alternately connected with one another inthe multistage distortion compensating section 20 and, for example, anumber of stages of the distortion compensating sections (distortioncompensators) 25 in the multistage distortion compensating section 20may be made the same as that of the transmission spans 100. However, thenumber of stages of the distortion compensating sections 25 may beeither larger or smaller than that of the transmission spans 100. Thedistortion compensating section 25 realizes highly accurate compensationwithout being limited to a reverse propagation form of the transmissionline. Thus, the number of stages of the distortion compensating sections25 may be set to an arbitrary value simply by taking balance betweencompensation accuracy and complexity imposed upon mounting intoconsideration, regardless of the actual configuration of thetransmission line.

In the case that the intensity of the optical signal to be input intothe chromatic dispersion compensating module 94 is high, the nonlineardistortion effect, the linear distortion effect and, then, the nonlineardistortion effect are sequentially generated in one transmission span100. Thus, in the above mentioned case, two nonlinear distortioncompensating sections 24 may be installed corresponding to thecharacteristics of the transmission span 100 used. That is, each one ormore linear distortion compensating sections 22 and nonlinear distortioncompensating sections 24 may be installed corresponding to the order inwhich the nonlinear distortion effect and the linear distortion effectare generated in a transmission span 100.

On the other hand, in the case that the power of the optical signal tobe input into the transmission line 90 is low or a fiber used as thetransmission line is of the kind having a low nonlinear coefficient, thenonlinear distortion effect generated in the transmission line 90 may bedisregarded. In the above mentioned case, only the linear distortioncompensating sections 22 may be installed corresponding to thecharacteristics of the transmission span 100 used, with no installationof the nonlinear distortion compensating section 24. In the case that arear excitation Raman amplifier is used as the optical amplifier, lightis incident from the output side of the transmission line 90 and hencenonlinear distortion effect is generated on both the output side and theinput side of the transmission line 90. In this case, more highlyaccurate compensation for the nonlinear distortion is realized byproviding three or more nonlinear distortion compensating sections 24corresponding to the characteristics of the transmission span 100 used.

As described above, combination of the linear distortion compensatingsections 22 with the nonlinear distortion compensating sections 24 maybe set so as to optimize distortion compensation performed using themultistage distortion compensating section 20, not limited to the abovementioned examples and not limited to a situation where the combinationis to be set so as to conform to the characteristics of the actuallyinstalled transmission line.

In the optical reception device illustrated in FIG. 8, an electricsignal which has been converted from an optical signal received from theoptical transmission line is input into the multistage distortioncompensating section 20. In the multistage distortion compensatingsection 20, a plurality of distortion compensating sections 25 arecascade-connected with one another. The distortion compensating section25 has the linear distortion compensating section 22 compensating forlinear waveform distortion exerted on an optical signal and thenonlinear distortion compensating section 24 compensating for nonlinearwaveform distortion exerted on the optical signal. Owing to the abovementioned arrangement, highly accurate distortion compensation isrealized. In addition, a distortion compensating section in which two ormore nonlinear distortion compensating sections 24 are installed and adistortion compensating section made up of only the linear distortioncompensating section 22 may be included in the multistage distortioncompensating section 20 as mentioned above.

FIG. 9 is a block diagram illustrating an example of the lineardistortion compensating section 22. In the example shown in FIG. 9, thenumber of stages of the multistage distortion compensating section 20 is“N” and the linear distortion compensating section 22 in the distortioncompensating section 25 at an n-th stage is illustrated. In FIG. 9, thelinear distortion compensating section 22 is an FIR (Finite ImpulseResponse) filter which includes a delay unit 32, an FIR coefficient 34,a multiplier 36 and an adder 38. The delay unit 32 delays each signal bya time τ. The multiplier 36 multiplies each delayed signal and an FIRcoefficient Ck(n). Here, “k” is the number of coefficients and one tofive coefficients are prepared in the example illustrated in FIG. 9. Thenumber of coefficients may be arbitrarily set. The adder 38 adds eachmultiplied signal.

The coefficient Ck(n) is calculated, for example, using a numericalformula 1.

$\begin{matrix}{C_{k} = {\frac{1}{2\pi}{\int_{- \pi}^{\pi}{{\exp\left\lbrack {{\frac{\mathbb{i}}{2}\left( \frac{\omega}{T_{s}} \right)^{2}\frac{cD}{2\pi\; f}} + {{\mathbb{i}}\;\omega\; k}} \right\rbrack}{\mathbb{d}\omega}}}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 1}\end{matrix}$

In the formula, “i” is an imaginary unit, “Ts” is a sampling timeinterval, “D” is a chromatic dispersion amount, “c” is a velocity oflight and “f” is a frequency of carrier light. For example, thechromatic dispersion amount D may be set first at its initial valueindicative of the amount of chromatic dispersion generated in acorresponding transmission span 100 and then may be adjusted so as tooptimize distortion compensation performed using the multistagedistortion compensating section 20.

FIG. 10 is a block diagram illustrating another example of the lineardistortion compensating section 22. In FIG. 10, the linear distortioncompensating section 22 includes a time-frequency domain convertingsection 40, a frequency domain linear distortion compensating section 42and a frequency-time domain converting section 44. The time-frequencydomain converting section 40 performs FFT (Fast Fourier Transform) on aninput signal to convert the signal to a signal indicative of a frequencydomain. The frequency domain linear distortion compensating section 42performs linear distortion compensation in the frequency domain. Thefrequency-time domain converting section 44 performs inverse FFT on thesignal so subjected to linear distortion compensation to convert thesignal to a signal indicative of a time domain. As mentioned above,linear distortion compensation may be performed in the frequency domain.Incidentally, as the time-frequency domain converting section 40, thefrequency domain linear distortion compensating section 42 and thefrequency-time domain converting section 44, components described, forexample, in “IEEE Communication Magazine, April 2002, pp 58-66” may beused.

As mentioned above, at least one of the linear distortion compensatingsections 22 may be formed as a linear distortion compensating sectionthat compensates for chromatic dispersion of the optical signal whichhas been output from the optical transmission line. In addition, an FIRfilter may be used as at least one of the linear distortion compensatingsections 22 as illustrated in FIG. 9. Likewise, an IIR (Infinite ImpulseResponse) filter may be used. Further, a frequency domain filter may beused as at least one of the linear distortion compensating sections 22as illustrated in FIG. 10.

FIG. 11 is a block diagram illustrating an example of the nonlineardistortion compensating section 24. FIG. 11 illustrates the nonlineardistortion compensating section 24 of the distortion compensatingsection 25 at an n-th (“n” is an arbitrary positive integer) stage. InFIG. 11, the nonlinear distortion compensating section 24 is a selfphase modulation compensator including the intensity monitor 70, thephase modulating section 72, a multiplier 76 and a nonlinear coefficient78. The multiplier 76 multiplies an output from the intensity monitor 70and the nonlinear coefficient b(n) 78 and outputs a result ofmultiplication to the phase modulating section 72. The arrangement otherthan the above is the same as that in FIG. 6 and hence descriptionthereof will be omitted.

As the nonlinear coefficient b(n) 78, for example, a coefficient (γ1)eff in the equation 5 described in “Optical Express 16” may be used. Inaddition, a coefficient b (n) determined by using the followingnumerical formula 2 obtained by further generalizing the above mentionedequation 5 may be used as an initial value and then the coefficient b(n)may be adjusted so as to optimize nonlinear distortion compensationperformed using the multistage distortion compensating section 20.b(n)=A∫ ₀ ¹γ(z)p(z)dz  Numerical Formula 2

In the formula, “A” is a constant, γ(z) is a coefficient of distortion,p(z) is an optical signal power, “z” is a position in a transmissionspan and “1” is a length of the transmission span.

As described above, at least some of the plurality of distortioncompensating sections 25 may compensate for waveform distortiongenerated in a transmission line in accordance with a coefficient whichhas been calculated in advance such as the FIR coefficient 34 of thelinear distortion compensating section 22 illustrated in FIG. 9 and thenonlinear coefficient b(n)78 of the nonlinear distortion compensatingsection 24 illustrated in FIG. 11.

In the above mentioned manner, at least one of the nonlinear distortioncompensating sections 24 may be formed as a nonlinear distortioncompensating section that compensates for phase modulation distortionexerted on an input signal based on the intensity of the input signal.The self phase modulation may be compensated for using the nonlineardistortion compensating section so formed.

FIG. 12 is a block diagram illustrating a digital processing sectionused in the case nonlinear distortion to be compensated for using themultistage distortion compensating section 20 is to be monitored. InFIG. 12, the digital processing section 50 includes the multistagedistortion compensating section 20, the signal processing section 10, asignal identifying section (signal decision section) 54, an NL(Non-Linear) monitoring section 56 and a CD (Chromatic Dispersion)monitoring section 58. A/D converted I-phase and Q-phase signals areinput into the digital processing section 50. The multistage distortioncompensating section 20 performs linear distortion compensation andnonlinear distortion compensation on the input signals. The signalprocessing section 10 processes the signals in the same manner as thatillustrated in FIG. 5. The signal decision section 54 identifies theprocessed signals. That is, the signal decision section 54 identifies asto whether the signal concerned is a control signal or a data signal.The NL monitoring section 56 monitors a nonlinear amount. The CDmonitoring section 58 monitors a chromatic dispersion amount.

An example of monitoring the nonlinear amount using the NL monitoringsection 56 will be described. FIG. 13 is a constellation diagramillustrating a QPSK (Quadrature Phase Shift Keying) operation by way ofexample. A ratio of a deviation σr in an amplitude direction of a symbolto a deviation σθ in a phase direction thereof is used as the nonlinearamount FNL as expressed by the following numerical formula 3.

$\begin{matrix}{F_{NL} = \frac{\sigma_{\theta}}{\sigma_{r}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 3}\end{matrix}$

In the formula, as the nonlinear amount FNL is increased, nonlinearityis increased accordingly. For example, in the case that a signal sKobtained after execution of phase synchronization is expressed by anumerical formula 4, a mean amplitude r, a mean phase θ, the standarddeviation σr in the amplitude direction and the standard deviation σθ inthe phase direction are respectively expressed by numerical formula 5 to8.s _(k) =I _(k) +jQ _(k)  Numerical Formula 4

In the formula, “Ik” is an amplitude of the I phase and “Qk” is anamplitude of the Q phase.

$\begin{matrix}{r = {\frac{1}{N}{\sum\limits_{k = 1}^{N}\sqrt{\left( {I_{k}^{2} + Q_{k}^{2}} \right)}}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 5} \\{\theta = {\frac{1}{N}{\sum\limits_{k = 1}^{N}{\tan^{- 1}\left( \frac{Q_{k}}{I_{k}} \right)}}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 6} \\{\theta = {\frac{1}{N}{\sum\limits_{k = 1}^{N}{\tan^{- 1}\left( \frac{Q_{k\;}}{I_{k}} \right)}}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 7} \\{\sigma_{\theta} = \sqrt{\frac{1}{N}{\sum\limits_{k = 1}^{N}\left( {{\tan^{- 1}\left( \frac{Q_{k}}{I_{k}} \right)} - \theta} \right)^{2}}}} & {{Numerical}\mspace{14mu}{Formula}\mspace{14mu} 8}\end{matrix}$

In the above mentioned manner, the nonlinear amount FNL may bemonitored.

The control section 30 may adjust the nonlinear coefficient b(n)illustrated in FIG. 11 based on the monitored nonlinear amount FNL. Forexample, the control section 30 may adjust the nonlinear coefficient soas to minimize the nonlinear amount FNL. The details will be describedin explanation of an embodiment.

FIG. 14 is a block diagram illustrating an example of an opticalreception device (optical receiver) according to an embodiment of thepresent invention. In FIG. 14, an optical signal which has been receivedfrom an optical transmission line is input into a polarization controlsection 60. The optical signal is, for example, an mPSK signal. Thepolarization control section 60 outputs an optical signal oriented in adesired polarizing direction based on the input optical signal. A 90°hybrid section 62 detects the optical signal based on an oscillatedoptical signal sent from a local optical transmitter (local opticaloscillator) 64 and outputs I-phase signal and Q-phase signal which areout of phase with each other by 90°. Each of photoelectric converters(O/Es) 66 converts the optical signal to an analog electric signal. Eachof ADCs 68 converts the analog electric signal to a digital electricsignal. The I-phase signal and the Q-phase signal which have beenconverted to digital electric signals are input into the digitalprocessing section 50. The digital processing section 50 is the same asthat illustrated in FIG. 12 and hence description thereof will beomitted. According to an embodiment of the present invention, aconfiguration in which the multistage distortion compensating section 20described in relation to FIG. 8 is installed may be implemented in anoptical reception device (optical receiver) of a digital form thatprocesses single polarization.

Next, an example of a polarization diversity type optical receptiondevice (a polarization diversity type optical receiver) according to anembodiment of the present invention will be described. FIG. 15A and FIG.15B are block diagrams illustrating the example of the optical receptiondevice according to an embodiment of the present invention. In FIG. 15A,an optical signal is input into a polarization beam splitter 61. Thepolarization beam splitter 61 divides the optical signal into opticalsignals oriented in two polarizing directions. A polarization beamsplitter 63 divides an oscillated optical signal sent from the localoptical oscillator 64 into optical signals oriented in two polarizingdirections. Respective polarized optical signals are converted intoI-phase and Q-phase digital electric signals using the 90° hybridsection 62, the O/E 66 and the ADC 68 and are input into a digitalprocessing section 50 a. As illustrated in FIG. 15B, the multistagedistortion compensating section 20, the signal processing section 10 andthe signal decision section (signal identifying section) 54 in thedigital processing section 50 a process I-phase and Q-phase signals inthe respective polarized signals. The arrangement other than the aboveis the same as that illustrated in FIG. 12 and hence description thereofwill be omitted. Incidentally, an NL monitoring section and a CDmonitoring section are not illustrated.

FIG. 16A and FIG. 16B are block diagrams illustrating an example of aself-coherent type optical reception device (a self-coherent typeoptical receiver) according to an embodiment of the present invention.In FIG. 16A, an input optical signal is branched into three opticalsignals using a beam splitter (not shown). Two optical signals sobranched are respectively input into delay interferometers 65. One ofthe delay interferometers 65 causes the optical signal to be subjectedto self-delayed interference to extract an I-phase signal included inthe optical signal. For example, the interferometer branches the inputsignal into two signals and delays one of the branched signals by onebit to make it interfere with another signal. Another delayinterferometer 65 causes the optical signal to be subjected toself-delayed interference to extract a Q-phase signal included in theoptical signal. The optical signals which haven been output from thedelay interferometers 65 are respectively input into the O/Es 66. Thelast one of three branched optical signals is input into itscorresponding O/E 66 as it is. The functions of the O/Es 66 and the ADCs68 have already been explained, that is, are the same as those describedin the above-described embodiment and hence description thereof will beomitted. Digital electric signals which have been output from the ADCs68 are input into a digital processing section 50 b.

As illustrated FIG. 16B, the digital processing section 50 b includes anelectric field reconstructing section 52, the multistage distortioncompensating section 20 and an MSPE (Multi Symbol Phase Estimation)section 55. The electric field reconstructing section 52 performsprocessing necessary for reconstructing a complex photoelectric fieldfrom information in the received signal and outputs signals so subjectedto processing as I-phase and Q-phase signals to the multistagedistortion compensating section 20. The multistage distortioncompensating section 20 performs distortion compensation on the electricsignals which have been input thereinto and outputs an electric signalwhich has been subjected to distortion compensation to the MSPE section55. The MSPE section 55 estimates a multi-symbol phase of the inputsignal and outputs a signal so estimated to a signal decision section(signal identifying section). The signal decision section identifies andoutputs the received signal. According to an embodiment of the presentinvention, the multistage distortion compensating section according toan embodiment may be used in a self-coherent type optical digitalreceiver.

FIG. 17A and FIG. 17B are block diagrams illustrating an example of apolarization diversity self-coherent type optical reception device (apolarization diversity self-coherent type optical receiver) according toan embodiment of the present invention. As illustrated in FIG. 17A, thepolarization beam splitter 61 divides an optical signal into opticalsignals oriented in two polarizing directions. The optical signals sodivided are processed using the delay interferometers 65, the O/Es 66and the ADCs 68 according to an embodiment illustrated in FIG. 16A andare input into a digital processing section 50 c. As illustrated in FIG.17B, the digital processing section 50 c includes two electric fieldreconstructing sections 52, the multistage distortion compensatingsection 20 and the MSPE section 55. Digital electric signalscorresponding to two optical signals which have been divided to beoriented in two polarizing directions are respectively input into theelectric field reconstructing sections 52. Each of the electric fieldreconstructing sections 52 performs a process of reconstructing acomplex photoelectric field in the same manner as that has already beenexplained in the description of the above-described embodiment andoutputs processed signals to the multistage distortion compensatingsection 20. The arrangement other than the above is the same as that inFIG. 16B that illustrates an embodiment and hence description thereofwill be omitted. According to an embodiment, a configuration having themultistage distortion compensating section 20 according to an embodimentmay be implemented in the polarization diversity self-coherent typeoptical reception device.

As another embodiment of the present invention, an example in whichmutual phase modulation compensation is performed as nonlineardistortion compensation will be described in an embodiment. Insubcarrier multiplexing multi-input and multi-output transmissionsystems, for example, such as an Orthogonal Frequency DivisionMultiplexing (OFDM) transmission system and a polarization multiplexingtransmission system, self phase modulation induced by a self channel (asubcarrier) and inter-channel (inter-subcarrier) mutual phase modulationinduced by different channels (for example, neighboring subcarriers) ina received optical signal are generated.

FIG. 18 is a block diagram illustrating a distortion compensatingsection 25 a forming one stage of the multistage distortion compensatingsection. The distortion compensating section 25 a compensates for mutualphase modulation between channels which are greatly mutuallyphase-modulated. Linear distortion compensating sections (lineardistortion compensators) 22 a and 22 b execute linear distortioncompensation respectively based on signals from input channels 1 and 2.A nonlinear distortion compensating section (nonlinear distortioncompensator) 24 a executes nonlinear distortion compensation processingbased on signals sent from the linear distortion compensating sections22 a and 22 b and outputs a processed signal to an output channel 1. Thenonlinear distortion compensating section (nonlinear distortioncompensator) 24 b executes nonlinear distortion compensation processingbased on signals sent from the linear distortion compensating sections22 a and 22 b and outputs a processed signal to an output channel 2.

FIG. 19 is a block diagram of the nonlinear distortion compensatingsection 24 a. In FIG. 19, the nonlinear distortion compensating section24 a has the intensity monitors 70, the nonlinear coefficients 78 andthe integrators 76 respectively corresponding to inputs 1 and 2, and anadder 79. In signals sent from the inputs 1 and 2, the coefficientb11(n) and the coefficient b12(n) are multiplied to their intensities asin the case in FIG. 11 illustrating an embodiment. The coefficientb11(n) or b12(n) is set in the same manner as that using the numericalformula 2. The adder 79 adds corresponding signals to the inputs 1 and 2and outputs a result of addition to a phase modulator 72. The phasemodulator 72 modulates the phase of the signal from the input 1 based onan output from the adder 79 and outputs the phase-modulated signal as anoutput 1. As described above, mutual phase modulation compensation maybe realized by performing distortion compensation on the signal from theinput 1 based on a signal obtained by multiplying the intensities of thesignals from the inputs 1 and 2 and nonlinear coefficients and addingtogether results of multiplication.

Although the above-described embodiment is of an example of performingmutual phase modulation compensation on two channels, mutual phasemodulation compensation may be also performed on three or more channelsaccording to an embodiment. As an alternative, all the nonlineardistortion compensating sections 24 of the multistage distortioncompensating section 20 illustrated in FIG. 8 may be formed as nonlineardistortion compensating sections that perform mutual phase modulationcompensation. Likewise, some of the nonlinear distortion compensatingsections 24 may be formed as nonlinear distortion compensating sectionsthat perform mutual phase modulation compensation. As mentioned above,at least one of the nonlinear distortion compensating sections 24 may beformed as a nonlinear distortion compensating section that compensatesfor phase modulation distortion exerted on one of a plurality of signalswhich have been input thereinto, based on the intensity of each of theplurality of signals. As a result, mutual phase modulation compensationmay be realized.

An embodiment is of an example of a system that performs control ofcoefficients of distortion compensation (for example, the coefficientCk(n) in FIG. 9, the coefficient b(n) in FIG. 11 and the coefficientsb11(n) and b12(n) in FIG. 19) used in the linear distortion compensatingsection 22 and the nonlinear distortion compensating section 24 in themultistage distortion compensating section 20. FIG. 20 is a diagramillustrating an optical transmission system according to an embodimentof the present invention. In FIG. 20, an optical transmission line card134 acting as an optical transmission device and an optical receptionline card 124 acting as an optical reception device (an opticalreceiver) are installed at both ends of an optical transmission line 132including transmission spans. In place of the optical reception linecard 124, an optical transmission/reception line card having an opticaltransmitting/receiving function may be installed. A network control unit128 is a computer that performs, for example, optical transmission linelink/network management and controls the operations of the opticaltransmission line card 134 and the optical reception line card 124. Amemory 130 is, for example, a transmission fiber/chromatic dispersioncompensation module database, and stores data sent from the networkcontrol unit 128 and outputs data to the network control unit 128.

The optical reception line card 124 has an optical receiving module 120and a line card control section 126 controlling the operation of theoptical receiving module 120. The optical receiving module 120 has thedigital processing section 50 and a module control section 122. Thedigital processing section 50 is formed, for example, by one chip. Theoptical receiving module 120 also has functional elements other than thedigital processing section 50, for example, illustrated in FIG. 14,though not illustrated in FIG. 20.

FIG. 21 is a diagram illustrating a manner of inputting information intothe digital processing section 50, the optical receiving module 120, theoptical reception line card 124, the network control unit 128 and thememory 130 and outputting information therefrom. The network controlunit 128 controls the memory 130 to store transmission lineconfiguration information. The transmission line configurationinformation is information on respective configurations of the opticaltransmission line 132, that is, information on, for example, the numberof spans, a length of each optical transmission line in eachtransmission span and kinds of the optical fiber, the optical amplifierand the chromatic dispersion compensating module which are used for datatransmission.

The network control unit 128 calculates setting transmission linephysical values based on the transmission line configuration informationand outputs the calculated physical values to the line card controlsection 126 of the optical reception line card 124. The settingtransmission line physical values are physical values used for settingcoefficients used in the multistage distortion compensating section 20,that is, physical values used to calculate, for example, thecoefficients Ck(n) and b(n) using the numerical formulae 1 and 2 suchas, for example, the distortion coefficient γ(z) and the optical signalpower p(z). These physical values are, for example, functions oftemperature. Thus, the network control unit 128 obtains information, forexample, on a current temperature of each transmission span andcalculates the distortion coefficient γ(z) and the optical signal powerp(z) based on the information.

The line card control section 126 of the optical reception line card 124calculates distortion compensation coefficients, that is, coefficientsused to perform distortion compensation based on the settingtransmission line physical values. The distortion compensationcoefficients are, for example, the coefficient Ck(n) in FIG. 9, thecoefficient b(n) in FIG. 11 and the coefficients b11(n) and b12(n) inFIG. 19. The line card control section 126 outputs the distortioncompensation coefficients to the module control section 122 of theoptical receiving module 120. The module control section 122 outputs thedistortion compensation coefficients to the multistage distortioncompensating section 20 of the digital processing section 50. Themultistage distortion compensating section 20 compensates for distortionbased on the distortion compensation coefficients.

The digital processing section 50 outputs residual distortioninformation to the module control section 122 of the optical receivingmodule 120. The residual distortion information is information onnonlinear distortion (for example, expressed by the numerical formula 3)or linear distortion which still remains after distortion compensationhas been performed using the multistage compensating section 20 and ismonitored using the NL monitoring section 56 and the CD monitoringsection 58 (see FIG. 12) of the digital processing section 50. Themodule control section 122 of the optical receiving module 120 outputsinformation which has been monitored using other circuits installed inthe optical receiving module 120 as module information to the line cardcontrol section 126 of the optical reception line card 124 together withthe residual distortion information. The line card control section 126of the optical reception line card 124 outputs information which hasbeen monitored using other circuits installed in the optical receptionline card 124 as transmission line quality information to the networkcontrol unit 128 together with the module information.

FIG. 22 illustrates a method of explaining a process performed using theline card control section 126. In FIG. 22, the line card control section126 performs initialization (operation S10). As described in relation toFIG. 21, the line card control section 126 calculates the distortioncompensation coefficients and applies the calculated distortioncompensation coefficients to the multistage distortion compensatingsection 20. The line card control section 126 obtains the residualdistortion information (operation S12). The line card control section126 judges whether the residual distortion is equal to or more than aspecified value (operation S14). When No, the process proceeds tooperation S20. When Yes, the line card control section 126 resets thedistortion compensation coefficients (operation S16). The line cardcontrol section 126 applies the distortion compensation coefficients tothe multistage distortion compensating section 20 (operation S16). Theline card control section 126 judges whether the process is terminated.When Yes, the process is terminated. When No, the process returns tooperation S12. Although an example in which the line card controlsection 126 calculates the distortion compensation coefficients isillustrated in FIG. 22, as an alternative, the module control section122 may calculate the distortion compensation coefficients or the linecard control section may calculates the nonlinear coefficient B(n) andthe chromatic dispersion amount D (see the numerical formula 1) and themodule control section 122 may calculate the linear coefficient Ck(n).

According to an embodiment, the line card control section 126 or themodule control section 122 changes a parameter (a coefficient ofdistortion compensation) used for at least one of distortioncompensating operations performed using the linear distortioncompensating section 22 and the nonlinear compensating section 24 basedon information on distortion remained in a signal to be output from themultistage distortion compensating sections 20. As a result, it maybecome possible to change the distortion compensation coefficient so asto reduce the residual distortion.

The preferred embodiment(s) of the present invention have been describedin detail. However, the present invention may not be limited to specificembodiments and the present invention may be modified and altered in avariety of ways within the scope of the gist of the present inventiondescribed in the appended claims.

Highly accurate nonlinear distortion compensation may be realized byusing the distortion compensator, the optical reception device, thedistortion compensator and optical reception device controlling methodsand the optical transmission system according to the embodiments of thepresent invention.

A compensator is provided with a configuration in which each of aplurality of distortion compensating sections has linear distortioncompensating sections compensating for linear waveform distortionexerted on the optical signal and nonlinear distortion compensatingsections compensating for nonlinear waveform distortion exerted on theoptical signal. The method and system disclosed herein includeselectively alternating between compensating for nonlinear distortionand linear distortion relative to an optical signal over plurality ofspans.

The embodiments can be implemented in computing hardware (computingapparatus) and/or software, such as (in a non-limiting example) anycomputer that can store, retrieve, process and/or output data and/orcommunicate with other computers. The results produced can be displayedon a display of the computing hardware. A program/software implementingthe embodiments may be recorded on computer-readable media comprisingcomputer-readable recording media. The program/software implementing theembodiments may also be transmitted over transmission communicationmedia. Examples of the computer-readable recording media include amagnetic recording apparatus, an optical disk, a magneto-optical disk,and/or a semiconductor memory (for example, RAM, ROM, etc.). Examples ofthe magnetic recording apparatus include a hard disk device (HDD), aflexible disk (FD), and a magnetic tape (MT). Examples of the opticaldisk include a DVD (Digital Versatile Disc), a DVD-RAM, a CD-ROM(Compact Disc-Read Only Memory), and a CD-R (Recordable)/RW. An exampleof communication media includes a carrier-wave signal.

Further, according to an aspect of the embodiments, any combinations ofthe described features, functions and/or operations can be provided.

Although a few embodiments have been shown and described, it would beappreciated by those skilled in the art that changes may be made inthese embodiments without departing from the principles and spirit ofthe invention, the scope of which is defined in the claims and theirequivalents.

What is claimed is:
 1. A distortion compensator inputting an electric signal obtained by photoelectric-converting an optical signal received from an optical transmission line, comprising: a plurality of distortion compensating sections, each having compensators configured to compensate for linear waveform distortion exerted on the optical signal; and at least one of the compensators configured to alternately compensate for nonlinear waveform distortion exerted on the optical signal, which are cascade-connected with one another, where a signal outputted is subjected to linear and non-linear waveform distortion compensation.
 2. The distortion compensator according to claim 1, wherein at least one of the linear distortion compensators compensates for chromatic dispersion of the optical signal.
 3. The distortion compensator according to claim 1, wherein at least one of the non-linear distortion compensators compensates for phase modulation distortion exerted on the input electric signal based on an intensity of the received optical signal and information obtained by arithmetically operating information with respect to the intensity.
 4. The distortion compensator according to claim 1, wherein at least one of the nonlinear distortion compensators inputs electric signals corresponding to a plurality of optical signals included in the optical signal to compensate for phase modulation distortion exerted on at least one of the plurality of signals based on the intensity of each of the plurality of signals.
 5. The distortion compensator according to claim 1, wherein the plurality of distortion compensators compensate for waveform distortion induced by the optical transmission line in accordance with a given coefficient.
 6. The distortion compensator according to claim 1, wherein the plurality of distortion compensators compensate for linear waveform distortion and nonlinear waveform distortion generated in a plurality of transmission spans of the optical transmission line.
 7. The distortion compensator according to claim 1, comprising: a control section changing a parameter used in at least one of distortion compensating operations performed using the linear distortion compensators and the nonlinear distortion compensators based on information on residual distortion in a signal output from the distortion compensator.
 8. An optical reception device, comprising: a distortion compensator inputting an electric signal obtained by photoelectric-converting an optical signal received from an optical transmission line, and wherein the distortion compensator includes a plurality of distortion compensators, at least one of the plurality of distortion compensators configured to compensate for linear waveform distortion exerted on the optical signal and alternatively compensate for nonlinear waveform distortion exerted on the optical signal, which are cascade-connected with one another, and where a signal outputted is subjected to linear and non-linear waveform distortion compensation.
 9. The optical reception device according to claim 8, wherein at least one of the linear distortion compensators compensates for chromatic dispersion of the optical signal.
 10. The optical reception device according to claim 8, wherein at least one of the nonlinear distortion compensators compensates for phase modulation distortion exerted on the input signal based on an intensity of the input signal.
 11. The optical reception device according to claim 8, wherein at least one of the nonlinear distortion compensators inputs electric signals corresponding to the plurality of optical signals included in the optical signal to compensate for the phase modulation distortion exerted on one of the plurality of signals based on the intensity of each of the plurality of signals.
 12. The optical reception device according to claim 8, wherein each of the plurality of distortion compensators compensates for the waveform distortion induced by the optical transmission line in accordance with a given coefficient.
 13. The optical reception device according to claim 8, wherein the plurality of distortion compensators compensate for linear waveform distortion and nonlinear waveform distortion generated in a plurality of transmission spans of the optical transmission line.
 14. The optical reception device according to claim 8, comprising: a control section changing a parameter used in at least one of distortion compensating operations performed using the linear distortion compensators and the nonlinear distortion compensators based on information on residual distortion in a signal output from the distortion compensator.
 15. A distortion compensator controlling method, comprising: a distortion compensating operation including: compensating for linear waveform distortion exerted on an optical signal; and alternately compensating for nonlinear waveform distortion exerted on the optical signal, respectively based on an electric signal obtained by photoelectric-converting the optical signal received from an optical transmission line; and executing the distortion compensating operation a plurality of times, where a signal outputted is subjected to linear and non-linear waveform distortion compensation.
 16. An optical reception device controlling method, an optical transmission system controlling a distortion compensator using the controlling method according to claim
 15. 17. A distortion compensator controlling method, comprising: a distortion compensating operation including: executing a linear distortion compensating operation compensating for linear waveform distortion exerted on an optical signal; and alternately executing a nonlinear distortion compensating operation compensating for nonlinear waveform distortion exerted on the optical signal, respectively based on an electric signal obtained by photoelectric-converting the optical signal received from an optical transmission line and information obtained from an optical transmission line channel network control unit; and executing the distortion compensating operation a plurality of times, where a signal outputted is subjected to linear and non-linear waveform distortion compensation.
 18. An optical reception device controlling method, comprising: compensating for linear waveform distortion exerted on an optical signal based on an electric signal obtained by photoelectric-converting the optical signal received from an optical transmission line; alternately compensating for nonlinear waveform distortion exerted on the optical signal based on the electric signal; and executing an operation of combining the linear waveform distortion compensating operation with the nonlinear waveform distortion compensating operation a plurality of times, where a signal outputted is subjected to linear and non-linear waveform distortion compensation.
 19. An optical transmission system controlling an optical reception device using the controlling method according to claim
 18. 20. An optical reception device controlling method, comprising: a distortion compensating operation including: compensating for linear waveform distortion exerted on an optical signal; and alternately compensating for nonlinear waveform distortion exerted on the optical signal, respectively based on an electric signal obtained by photoelectric-converting the optical signal received from an optical transmission line and information obtained from an optical transmission line network control unit; and executing the distortion compensating operation a plurality of times, where a signal outputted is subjected to linear and non-linear waveform distortion compensation. 